Apparatus for delivering gas and illumination source for generating high harmonic radiation

ABSTRACT

Disclosed is a high harmonic generation (HHG) radiation source which may be used to generate measurement radiation for an inspection apparatus. In such a radiation source, a pump radiation source is operable to emit pump radiation at a high harmonic generation gas medium thereby exciting the high harmonic generation gas medium within a pump radiation interaction region so as to generate the high harmonic radiation and an ionization radiation source is operable to emit ionization radiation at the high harmonic generation gas medium to ionize a gas at an ionization region between the pump radiation interaction region and an optical output of the illumination source.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 15/903,392, filed on Feb. 23, 2018, which claims priority to EPPatent Application No. 17160996, filed on Mar. 15, 2017, EP PatentApplication No. 17175640, filed on Jun. 13, 2017, and EP PatentApplication No. 17189172, filed on Sep. 4, 2017, which are incorporatedherein by references in their entireties.

FIELD

The present invention relates to a gas delivery apparatus, and inparticular to a gas delivery apparatus for use in an illumination orradiation system.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Multiple layers, each having a particular pattern and materialcomposition, are applied to define functional devices andinterconnections of the finished product.

In lithographic processes, it is desirable frequently to makemeasurements of the structures created, e.g., for process control andverification. Various tools for making such measurements are known,including scanning electron microscopes, which are often used to measurecritical dimension (CD), and specialized tools to measure overlay, theaccuracy of alignment of two layers in a device. Recently, various formsof scatterometers have been developed for use in the lithographic field.

Examples of known scatterometers often rely on provision of dedicatedmetrology targets. For example, a method may require a target in theform of a simple grating that is large enough that a measurement beamgenerates a spot that is smaller than the grating (i.e., the grating isunderfilled). In so-called reconstruction methods, properties of thegrating can be calculated by simulating interaction of scatteredradiation with a mathematical model of the target structure. Parametersof the model are adjusted until the simulated interaction produces adiffraction pattern similar to that observed from the real target.

In addition to measurement of feature shapes by reconstruction,diffraction-based overlay can be measured using such apparatus, asdescribed in published patent application US2006066855A1.Diffraction-based overlay metrology using dark-field imaging of thediffraction orders enables overlay measurements on smaller targets.These targets can be smaller than the illumination spot and may besurrounded by product structures on a wafer. Examples of dark fieldimaging metrology can be found in numerous published patentapplications, such as for example US2011102753A1 and US20120044470A.Multiple gratings can be measured in one image, using a compositegrating target. The known scatterometers tend to use light in thevisible or near-IR wave range, which requires the pitch of the gratingto be much coarser than the actual product structures whose propertiesare actually of interest. Such product features may be defined usingdeep ultraviolet (DUV) or extreme ultraviolet (EUV) radiation having farshorter wavelengths. Unfortunately, such wavelengths are not normallyavailable or usable for metrology.

On the other hand, the dimensions of modern product structures are sosmall that they cannot be imaged by optical metrology techniques. Smallfeatures include for example those formed by multiple patterningprocesses, and/or pitch-multiplication. Hence, targets used forhigh-volume metrology often use features that are much larger than theproducts whose overlay errors or critical dimensions are the property ofinterest. The measurement results are only indirectly related to thedimensions of the real product structures, and may be inaccurate becausethe metrology target does not suffer the same distortions under opticalprojection in the lithographic apparatus, and/or different processing inother steps of the manufacturing process. While scanning electronmicroscopy (SEM) is able to resolve these modern product structuresdirectly, SEM is much more time consuming than optical measurements.Moreover, electrons are not able to penetrate through thick processlayers, which makes them less suitable for metrology applications. Othertechniques, such as measuring electrical properties using contact padsis also known, but it provides only indirect evidence of the trueproduct structure.

By decreasing the wavelength of the radiation used during metrology(i.e. moving towards the “soft X-ray” wavelength spectrum), it ispossible to resolve smaller structures, to increase sensitivity tostructural variations of the structures and/or penetrate further intothe product structures. One such method of generating suitably highfrequency radiation (e.g., soft X-ray and/or EUV radiation) is by usinga high harmonic generation (HHG) radiation source. Such a HHG radiationsource uses laser radiation (e.g., infra-red radiation) to excite a HHGgenerating medium, thereby generating high harmonics comprising highfrequency radiation.

One problem with generated high frequency radiation is that it isabsorbed by any particles present in its path. This requires that theHHG radiation source be kept at near vacuum. Since the HHG generatingmedium is typically a gas, the generating medium must be carefullycontrolled to prevent it from absorbing the generated radiation.

A further problem with HHG radiation sources is to maintain a stableoutput of generated radiation. Any fluctuations in the supply of the HHGgenerating medium may negatively impact the temporal stability of theradiation output.

SUMMARY

In accordance with a first aspect of the invention, there is provided anillumination source for generating high harmonic radiation, comprising:a pump radiation source operable to emit pump radiation at a highharmonic generation gas medium thereby exciting the high harmonicgeneration gas medium within a pump radiation interaction region so asto generate the high harmonic radiation; and an ionization radiationsource operable to emit ionization radiation at the high harmonicgeneration gas medium to ionize a gas at an ionization region betweenthe pump radiation interaction region and an optical output of theillumination source.

In some embodiments, the ionization region is adjacent to the pumpradiation interaction region.

In some embodiments, the illumination source is arranged such that theionization radiation ionizes the gas at the ionization regionsubstantially simultaneously with the pump radiation exciting the highharmonic generation gas medium.

In some embodiments, a propagation direction of the ionization radiationis opposite to a propagation direction of the pump radiation.

In some embodiments, a propagation direction of the ionization radiationis orthogonal to a propagation direction of the pump radiation.

In some embodiments, the pump radiation source and the ionizationradiation source are located on opposite sides of the high harmonicgeneration gas medium.

In some embodiments, at least one of wavelength, polarization andpropagation angle of the pump radiation is different to that of theionization radiation, thereby enabling a separation of returning pumpradiation and/or ionizing radiation.

In some embodiments, the illumination source further comprises anoptical element operable to focus the ionization radiation on a linefocus at the ionization region.

In some embodiments, the illumination source further comprises a commonoscillator operable to provide both the pump radiation and ionizationradiation.

In some embodiments, the optical output comprises an aperture arrangedto allow the pump radiation to pass and to block the high harmonicradiation.

In some embodiments, the illumination source further comprises a dosecontrol operable to control an absorption of the ionization radiationwithin the ionization region by controlling a timing of the ionizationradiation with respect to the pump radiation, and/or an intensity of theionization radiation.

In accordance with a second aspect of the invention, there is providedan illumination source for generating high harmonic radiation,comprising: a pump radiation source operable to emit pump radiation; anda gas delivery system operable to generate a gas flow including a firstgas; wherein the gas delivery system further comprises an optical pathto allow the pump radiation pass through the gas flow to excite thefirst gas to generate the high harmonic radiation.

In some embodiments, the illumination source further comprises the gasdelivery system is further operable to generated the gas flow includinga second gas configured to modify a density profile of the first gas.

In some embodiments, the gas delivery system comprises: a first gasdelivery element operable to emit the first gas; and a second gasdelivery element operable to emit the second gas; wherein the first gasdelivery element and the second gas delivery element are arrangedconcentrically.

In some embodiments, the gas delivery system further comprises afiltering element operable to provide a laminar flow profile of the gasflow.

In some embodiments, the gas delivery system further comprises a gas jetshaping device operable to enlarge a pump radiation interaction regionwhere pump radiation interacts with the first gas.

In some embodiments, the illumination source further comprises the gasjet shaping device comprises a curved wall attached adjacent to a gasoutput of the gas flow, and an optical aperture on the curved wall.

In some embodiments, the gas delivery system further comprises atoroidal gas delivery component configured to deliver the gas flow tothe optical path.

In some embodiments, the gas delivery system further comprises a gasbuffer element operable to maintain the gas flow substantially constant.

In some embodiments, the gas buffer element comprises a temperaturecontrolling element operable to control a temperature of the gas flow.

Further aspects, features and advantages of the invention, as well asthe structure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus;

FIG. 2 depicts a lithographic cell or cluster in which an inspectionapparatus according to the present invention may be used;

FIGS. 3(a) and 3(b) illustrate schematically an inspection apparatusadapted to perform known dark-field imaging inspection methods;

FIG. 4 schematically illustrates a metrology apparatus using a HHGsource adaptable according to an embodiment of the invention;

FIG. 5 shows schematically details of a HHG gas cell usable in a HHGsource;

FIG. 6 shows a first exemplary gas delivery element usable in a HHG gascell;

FIGS. 7(a)-7(e) illustrate the principle of the gas delivery elementaccording to the present invention;

FIG. 8 shows a second exemplary gas delivery element according to thepresent invention;

FIG. 9 shows a HHG radiation source according to an embodiment of thepresent invention; and

FIGS. 10(a) and 10(b) show a filter component according to an embodimentof the present invention;

FIGS. 11(a) and 11(b) show a gas delivery element according to anembodiment of the present invention comprising a gas jet shaping device;

FIG. 12 is a plot of number density of the emitted gas against distancealong the optical path for the gas delivery element of FIG. 11 and a gasdelivery element without a gas jet shaping device;

FIGS. 13(a)-13(d) show schematically, four steps of a method forgenerating soft X-ray measurement radiation using a plasma to mitigatereabsorption;

FIG. 14 shows an arrangement for performing the method shown in FIG. 13;

FIGS. 15(a)-15(c) show schematically, three steps of an alternativemethod for generating soft X-ray measurement radiation using a plasma tomitigate reabsorption; and

FIGS. 16(a) and 16(b) show (a) a further arrangement for mitigatingreabsorption of generated measurement radiation; and (b) a plot of gasdensity against the pump radiation propagation direction x, illustratingthe working principle behind the arrangement of FIG. 16(a).

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Before describing embodiments of the invention in detail, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g., UV radiation or DUV radiation), a patterningdevice support or support structure (e.g., a mask table) MT constructedto support a patterning device (e.g., a mask) MA and connected to afirst positioner PM configured to accurately position the patterningdevice in accordance with certain parameters; two substrate tables(e.g., a wafer table) WTa and WTb each constructed to hold a substrate(e.g., a resist coated wafer) W and each connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and a projection system (e.g., arefractive projection lens system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g., including one or more dies) of the substrate W. Areference frame RF connects the various components, and serves as areference for setting and measuring positions of the patterning deviceand substrate and of features on them.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support MT may be a frame or a table, for example,which may be fixed or movable as required. The patterning device supportmay ensure that the patterning device is at a desired position, forexample with respect to the projection system.

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive patterning device). Alternatively, theapparatus may be of a reflective type (e.g., employing a programmablemirror array of a type as referred to above, or employing a reflectivemask). Examples of patterning devices include masks, programmable mirrorarrays, and programmable LCD panels. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.” The term “patterning device” can also beinterpreted as referring to a device storing in digital form patterninformation for use in controlling such a programmable patterningdevice.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems.

In operation, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may for example include an adjuster AD for adjustingthe angular intensity distribution of the radiation beam, an integratorIN and a condenser CO. The illuminator may be used to condition theradiation beam, to have a desired uniformity and intensity distributionin its cross section.

The radiation beam B is incident on the patterning device MA, which isheld on the patterning device support MT, and is patterned by thepatterning device. Having traversed the patterning device (e.g., mask)MA, the radiation beam B passes through the projection system PS, whichfocuses the beam onto a target portion C of the substrate W. With theaid of the second positioner PW and position sensor IF (e.g., aninterferometric device, linear encoder, 2-D encoder or capacitivesensor), the substrate table WTa or WTb can be moved accurately, e.g.,so as to position different target portions C in the path of theradiation beam B. Similarly, the first positioner PM and anotherposition sensor (which is not explicitly depicted in FIG. 1) can be usedto accurately position the patterning device (e.g., mask) MA withrespect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan.

Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g., mask) MA, the mask alignment marks may be located betweenthe dies. Small alignment mark may also be included within dies, inamongst the device features, in which case it is desirable that themarkers be as small as possible and not require any different imaging orprocess conditions than adjacent features. The alignment system, whichdetects the alignment markers, is described further below.

The depicted apparatus could be used in a variety of modes. In a scanmode, the patterning device support (e.g., mask table) MT and thesubstrate table WT are scanned synchronously while a pattern imparted tothe radiation beam is projected onto a target portion C (i.e., a singledynamic exposure). The speed and direction of the substrate table WTrelative to the patterning device support (e.g., mask table) MT may bedetermined by the (de-)magnification and image reversal characteristicsof the projection system PS. In scan mode, the maximum size of theexposure field limits the width (in the non-scanning direction) of thetarget portion in a single dynamic exposure, whereas the length of thescanning motion determines the height (in the scanning direction) of thetarget portion. Other types of lithographic apparatus and modes ofoperation are possible, as is well-known in the art. For example, a stepmode is known. In so-called “maskless” lithography, a programmablepatterning device is held stationary but with a changing pattern, andthe substrate table WT is moved or scanned.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Lithographic apparatus LA is of a so-called dual stage type which hastwo substrate tables WTa, WTb and two stations—an exposure station EXPand a measurement station MEA—between which the substrate tables can beexchanged. While one substrate on one substrate table is being exposedat the exposure station, another substrate can be loaded onto the othersubstrate table at the measurement station and various preparatory stepscarried out. This enables a substantial increase in the throughput ofthe apparatus. The preparatory steps may include mapping the surfaceheight contours of the substrate using a level sensor LS and measuringthe position of alignment markers on the substrate using an alignmentsensor AS. If the position sensor IF is not capable of measuring theposition of the substrate table while it is at the measurement stationas well as at the exposure station, a second position sensor may beprovided to enable the positions of the substrate table to be tracked atboth stations, relative to reference frame RF. Other arrangements areknown and usable instead of the dual-stage arrangement shown. Forexample, other lithographic apparatuses are known in which a substratetable and a measurement table are provided. These are docked togetherwhen performing preparatory measurements, and then undocked while thesubstrate table undergoes exposure.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include spin coaters SCto deposit resist layers, developers DE to develop exposed resist, chillplates CH and bake plates BK. A substrate handler, or robot, RO picks upsubstrates from input/output ports I/O1, I/O2, moves them between thedifferent process apparatus and delivers then to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. The supervisory control system may also control one or moreinspection apparatuses MET, used to perform measurements on substrates Wto ensure quality and consistency of the lithographic process, and todetermine any necessary corrections. Thus, the different apparatus canbe operated to maximize throughput and processing efficiency. Thesubstrates processed by the track are then transferred to otherprocessing tools for etching and other chemical or physical treatmentswithin the device manufacturing process.

The lithographic apparatus control unit LACU controls all the movementsand measurements of the various actuators and sensors described. LACUalso includes signal processing and data processing capacity toimplement desired calculations relevant to the operation of theapparatus. In the terminology of the introduction and claims, thecombination of these processing and control functions referred to simplyas the “controller”. In practice, control unit LACU will be realized asa system of many sub-units, each handling the real-time dataacquisition, processing and control of a subsystem or component withinthe apparatus. For example, one processing subsystem may be dedicated toservo control of the substrate positioner PW. Separate units may evenhandle coarse and fine actuators, or different axes. Another unit mightbe dedicated to the readout of the position sensor IF. Overall controlof the apparatus may be controlled by a central processing unit,communicating with these sub-systems processing units, with operatorsand with other apparatuses involved in the lithographic manufacturingprocess.

FIG. 3(a) shows schematically the key elements of an inspectionapparatus MET implementing so-called dark field imaging metrology. Theapparatus may be a stand-alone device or incorporated in either thelithographic apparatus LA, e.g., at the measurement station, or thelithographic cell LC. An optical axis, which has several branchesthroughout the apparatus, is represented by a dotted line O. A targetgrating structure T and diffracted rays are illustrated in more detailin FIG. 3(b).

As described in the prior applications cited in the introduction, thedark-field-imaging apparatus of FIG. 3(a) may be part of a multi-purposeangle-resolved scatterometer that may be used instead of or in additionto a spectroscopic scatterometer. In this type of inspection apparatus,radiation emitted by a radiation source 11 (in this disclosure a HHGradiation source) is conditioned by an illumination system 12. Forexample, illumination system 12 may include a collimating lens system, acolor filter, a polarizer and an aperture device. The conditionedradiation follows an illumination path, in which it is reflected bypartially reflecting surface 15 and focused into a spot S on substrate Wvia a microscope objective lens 16. A metrology target T may be formedon substrate W. Lens 16, has a high numerical aperture (NA), preferablyat least 0.9 and more preferably at least 0.95. Immersion fluid can beused to obtain with numerical apertures over 1 if desired. Themulti-purpose scatterometer may have two or more measurement branches.Additionally, further optical systems and branches will be included in apractical apparatus, for example to collect reference radiation forintensity normalization, for coarse imaging of capture targets, forfocusing and so forth. Details of these can be found in the priorpublications mentioned above. For the purposes of the presentdisclosure, only the measurement branch of interest for the dark-filedimaging metrology is illustrated and described in detail.

In the collection path for dark-field imaging, imaging optical system 21forms an image of the target on the substrate W on sensor 23 (e.g. a CCDor CMOS sensor). An aperture stop 20 is provided in a plane P′ in thecollection path. Plane P′ is a plane conjugate to a pupil plane P″ ofobjective lens 16. Aperture stop 20 may also be called a pupil stop.Aperture stop 20 can take different forms, just as the illuminationaperture can take different forms. The aperture stop 20, in combinationwith the effective aperture of lens 16, determines what portion of thescattered radiation is used to produce the image on sensor 23.Typically, aperture stop 20 functions to block the zeroth orderdiffracted beam so that the image of the target formed on sensor 23 isformed only from the first order beam(s). In an example where both firstorder beams are combined to form an image, this would be the so-calleddark field image, equivalent to dark-field microscopy. In the presentapplication, however, only one of the first orders is imaged at a time,as explained below. The images captured by sensor 23 are output to imageprocessor and controller 40, the function of which will depend on theparticular type of measurements being performed. For the presentpurpose, measurements of asymmetry of the target structure areperformed. Asymmetry measurements can be combined with knowledge of thetarget structures to obtain measurements of performance parameters oflithographic process used to form them. Performance parameters that canbe measured in this way include for example overlay, focus and dose.

Where a metrology target T is provided on substrate W, this may be a 1-Dgrating, which is printed such that after development, the bars areformed of solid resist lines. The target may be a 2-D grating, which isprinted such that after development, the grating is formed of solidresist pillars or vias in the resist. The bars, pillars or vias mayalternatively be etched into the substrate. Each of these gratings is anexample of a target structure whose properties may be investigated usingthe inspection apparatus.

The various components of illumination system 12 can be adjustable toimplement different metrology ‘recipes’ within the same apparatus. Inaddition to selecting wavelength (color) and polarization ascharacteristics of the particular, illumination system 12 can beadjusted to implement different illumination profiles. Because plane P″is conjugate with pupil plane P′ of objective lens 16 and the plane ofthe detector 23, an illumination profile in plane P″ defines the angulardistribution of light incident on substrate W in spot S. To implementdifferent illumination profiles, an aperture device can be provided inthe illumination path. The aperture device may comprise differentapertures mounted on a movable slide or wheel. It may alternativelycomprise a programmable spatial light modulator. As a furtheralternative, optical fibers may be disposed at different locations inthe plane P″ and used selectively to deliver light or not deliver lightat their respective locations. These variants are all discussed andexemplified in the documents cited above.

In a first example illumination mode, rays 30 a are provided so that theangle of incidence is as shown at ‘I’ and the path of the zero order rayreflected by target T is labeled ‘0’ (not to be confused with opticalaxis ‘O’). In a second illumination mode, rays 30 b can be provided, inwhich case the angles of incidence and reflection will be swapped. Bothof these illumination modes will be recognized as off-axis illuminationmodes. Many different illumination modes can be implemented fordifferent purposes.

As shown in more detail in FIG. 3(b), target grating T as an example ofa target structure is placed with substrate W normal to the optical axisO of objective lens 16. In the case of an off-axis illumination profile,a ray of illumination I impinging on grating T from an angle off theaxis O gives rise to a zeroth order ray (solid line 0) and two firstorder rays (dot-chain line +1 and double dot-chain line −1). It shouldbe remembered that with an overfilled small target grating, these raysare just one of many parallel rays covering the area of the substrateincluding metrology target grating T and other features. Since the beamof illuminating rays 30 a has a finite width (necessary to admit auseful quantity of light), the incident rays I will in fact occupy arange of angles, and the diffracted rays 0 and +1/−1 will be spread outsomewhat. According to the point spread function of a small target, eachorder +1 and −1 will be further spread over a range of angles, not asingle ideal ray as shown.

Referring also to FIG. 3(a), under the first illumination mode with rays30 a, +1 order diffracted rays from the target grating will enter theobjective lens 16 and contribute to the image recorded at sensor 23.When the second illumination mode is used, rays 30 b are incident at anangle opposite to rays 30 b, and so the −1 order diffracted rays enterthe objective and contribute to the image. Aperture stop 20 blocks thezeroth order radiation when using off-axis illumination. As described inthe prior publications, illumination modes can be defined with off-axisillumination in X and Y directions.

By comparing images of the target grating under these differentillumination modes, asymmetry measurements can be obtained.Alternatively, asymmetry measurements could be obtained by keeping thesame illumination mode, but rotating the target. While off-axisillumination is shown, on-axis illumination of the targets may insteadbe used and a modified, off-axis aperture 20 could be used to passsubstantially only one first order of diffracted light to the sensor. Ina further example, prisms are used in place of aperture stop 20 whichhave the effect of diverting the +1 and −1 orders to different locationson sensor 23 so that they can be detected and compared without the needfor two sequential image capture steps. This technique is disclosed inthe above-mentioned published patent application US2011102753A1, thecontents of which are hereby incorporated by reference. 2nd, 3rd andhigher order beams (not shown in FIG. 3) can be used in measurements,instead of or in addition to the first order beams. As a furthervariation, the off-axis illumination mode can be kept constant, whilethe target itself is rotated 180 degrees beneath objective lens 16 tocapture images using the opposite diffraction orders.

The above techniques are typically performed using radiation with avisible wavelength. As such, the scatterometry targets have a pitch thatis larger than that of the product structures on the substrate. As anexample, a scatterometry target may have a target grating pitch measuredin microns (μm), whereas product structures on the same substrate mayhave a pitch measured in nanometers (nm).

This difference in pitch induces an offset between the measured overlayand the actual overlay on the product structures. The offset is at leastpartly due to optical projection distortions in the lithographicapparatus and/or different processing in other steps of themanufacturing process. Presently, the offset comprises a significantcontribution to the overall measured overlay. Reducing or eliminating itwill therefore improve overall overlay performance.

Metrology tools may be developed which use sources that emit radiationin “soft X-ray” or EUV range, for example having wavelengths in therange from 0.1 to 100 nm, or, optionally, in the range from 1 to 50 nm,or optionally, in the wavelength range from 10 to 20 nm. Examples ofsuch sources include Discharge Produced Plasma sources, Laser ProducedPlasma Sources or High-order Harmonic Generation (HHG) sources. HHGsources are known to be able to provide large flux of collimated photons(high luminance) in the emitted light.

HHG sources used in metrology applications are illustrated and furtherdescribed in the European patent applications EP152020301, EP16168237,EP16167512, which are hereby incorporated in their entirety byreference. In metrology applications, such HHG sources may be used (forexample) in normal incidence, very close to normal incidence (e.g.,within 10 degrees from normal), at a grazing incidence (e.g., within 20degrees from surface), at an arbitrary angle or at multiple angles (toobtain more measurement information in a single capture).

FIG. 4 illustrates a metrology arrangement showing the radiation source430 in more detail. Radiation source 430 is an HHG source for generating“soft X-ray”/EUV (high harmonic) radiation based on high harmonicgeneration (HHG) techniques. Main components of the radiation source 430are a pump radiation source 431 (e.g. a pump laser or oscillator) and anHHG medium, such as a HHG gas cell 432. A gas supply 434 suppliessuitable gas to the gas cell, where it is optionally ionized by anelectric source (not shown). The pump radiation source 431 may be forexample a fiber-based laser with an optical amplifier, producingradiation pulses of infrared radiation lasting less than 1 ns (1nanosecond) per pulse, with a pulse repetition rate up to severalmegahertz, as required. The wavelength of the pump radiation may be forexample in the region of 1 μm (1 micron). The radiation pulses aredelivered as a pump radiation beam 440 to the HHG gas cell 432, where aportion of the radiation is converted to higher frequencies. From theHHG gas cell 432 emerges a beam of measurement radiation 442 includingcoherent radiation of the desired wavelength or wavelengths.

The measurement radiation 442 may contain multiple wavelengths. If theradiation is also monochromatic, then measurement calculations(reconstruction) may be simplified, but it is easier with HHG to produceradiation with several wavelengths. These are matters of design choice,and may even be selectable options within the same apparatus. Differentwavelengths will, for example, provide different levels of contrast whenimaging structure of different materials. For inspection of metalstructures or silicon structures, for example, different wavelengths maybe selected to those used for imaging features of (carbon-based) resist,or for detecting contamination of such different materials.

One or more filtering devices 444 may be provided. For example a filtersuch as a thin membrane of Aluminum (Al) may serve to cut thefundamental IR radiation from passing further into the inspectionapparatus. A grating may be provided to select one or more specificharmonic wavelengths from among those generated in the gas cell 432.Some or all of the beam paths may be contained within a vacuumenvironment, bearing in mind that EUV radiation is absorbed whentraveling in air. The various components of radiation source 430 andillumination optics can be adjustable to implement different metrology‘recipes’ within the same apparatus. For example different wavelengthsand/or polarization can be made selectable.

From the radiation source 430, the filtered measurement beam enters aninspection chamber where the substrate W including a structure ofinterest or target structure is held for inspection by substrate support414. The target structure is labeled T. The atmosphere within inspectionchamber is maintained near vacuum by vacuum pump 452, so that the softX-ray radiation can pass without undue attenuation through theatmosphere. The illumination system includes one or more opticalelements 454 for focusing the radiation into a focused beam 456, and maycomprise for example a two-dimensionally curved mirror, or a series ofone-dimensionally curved mirrors, as described in the prior applicationsmentioned above. Diffraction gratings such as the spectroscopic gratingscan be combined with such mirrors, if desired. The focusing is performedto achieve a round or elliptical spot under 10 μm in diameter, whenprojected onto the structure of interest. Substrate support 414comprises for example an X-Y translation stage and a rotation stage, bywhich any part of the substrate W can be brought to the focal point ofbeam to in a desired orientation. Thus the radiation spot S is formed onthe structure of interest. The radiation scattered 408 from thestructure of interest is then detected by detector 460.

FIG. 5 shows a more detailed illustration of an exemplary HHG gas cell,such as may for example be implemented in the system described in FIG.4. For ease of comparison with FIG. 4, elements of FIG. 5 similar tocorresponding elements of FIG. 4 are labelled with reference signssimilar to those used in FIG. 4, but with prefix “5” instead of “4”.

Shown is the incoming pump radiation 540 (e.g IR radiation), focused onHHG gas cell 532 (or other HHG generating medium). Shown beyond the HHGgas cell 532 is the generated HHG radiation (measurement radiation orsoft X-ray/EUV radiation) 542 and the remaining pump radiation 544,which needs to be filtered out of the generated measurement radiation542. As there is typically a region of overlap of the generatedmeasurement radiation 542 and the remaining pump radiation 544, a filterwhich is largely transparent to the measurement radiation 542, but whichblocks the pump radiation 544 is required. This may in some examples besolved by using ultra-thin metal film filters. Within the HHG gas cell,a capillary tube 570 delivers gas (i.e. the HHG medium) into theinspection chamber (as indicated by arrow 572). Once the gas exits thecapillary tube, it spreads into the inspection chamber dependent on anumber of characteristics (e.g. velocity, material properties of thefluid and properties of the capillary tube), thereby forming a “cloud”of gas 574. The pump radiation propagates through the gas, therebygenerating the measurement radiation. As described above, the inspectionchamber is maintained near vacuum to avoid the generated soft X-rayradiation being absorbed by the presence of atmospheric particles.

One problem with the known exemplary arrangement is that gas particlesspread substantially in all directions once they exit the capillarytube. While the gas particles are necessary to generate the measurementradiation, as described above, any gas particles present in the path ofthe measurement radiation will absorb the measurement radiation. As asubstantial percentage of the gas particles may propagate along orthrough the path of the measurement radiation, a substantial portion ofthe measurement radiation may be absorbed in the known system. In otherterms, gas particles in the path of the measurement radiation maynegatively affect the intensity and the temporal stability of themeasurement radiation.

The inventors have realized that it is possible to provide an apparatusthat delivers gas, while minimizing the amount of absorption of thegenerated radiation.

FIG. 6 illustrates a gas delivery system 600 for use in an illuminationsource in accordance with an aspect of the present invention. The gasdelivery system may be implemented in a suitable radiation orillumination system, such as the one shown in FIG. 4 above.

The system comprises a gas delivery element 602 that is arranged todirect gas in at least a first direction 603. The gas delivery elementcomprises an optical input 606. Additionally, the gas delivery elementcomprises an optical output 608. The optical input and the optical inputdefine an optical path 610 that is oriented in a second direction, thesecond direction being non-perpendicular and non-parallel to the firstdirection. The optical input and/or the optical output may in someexamples be dependent on the characteristics of the pump radiation beam.In one example, the optical input and optical output are matched tosubstantially conform to the beam shape of the pump radiation.

The gas delivery element may have any suitable or convenient shape. Inthe present examples, the gas delivery element comprises a two pairs ofopposed walls; a first wall 612 that comprises the optical input 606; asecond wall 614 that comprises the optical output; a first side wall616; and a second side wall 618.

During operation, gas 604 enters the gas delivery element and flowssubstantially in the first direction 603. A first portion of the gas 620will escape through the optical input 606, and a second portion of thegas 622 will escape through the optical output 608. As explained above,the second portion of the gas will serve to absorb the generatedradiation, which reduces the intensity of the generated radiation. Athird portion 605 of the gas is directed towards an output of the gasdelivery element.

The gas delivery system is arranged so that the pump radiation beampropagates along the optical path 610, and so that the pump radiationbeam is focused at a point that is substantially inside the gas deliverysystem (i.e. located on the optical path 610 and between the first wall612 and the second wall 614). In other terms, the pump radiation beam isarranged so as to maximize the intensity of the pump radiation (i.e.having a focus point or “beam waist”) positioned within the gas flowinside the gas delivery element. It will, however, be appreciated thatthis is for exemplary purposes only. In some examples, the focus pointis positioned on the optical path, but is not located inside the gasdelivery element. In an example, the focus point is positioned on theinput side of the gas delivery element. In another example, the focuspoint is positioned on the output side of the gas delivery element.

It will, of course, be appreciated that a plurality of specificcross-sections and/or shapes of the gas delivery element may beenvisaged. In the present example, the gas delivery element is comprisedof two pairs of opposed walls that define a specific cross-section forthe gas to flow within. It will be appreciated that a number of specificcross-sections (including, but not limited to: circular; elliptical; orrectangular) may be envisaged. In some examples, the cross-section ofthe gas delivery element may be adapted to provide one or severalspecific effect, e.g. specific flow profiles of the gas.

FIG. 7 illustrates schematically the principle of the present invention.For ease of comparison with FIG. 6, elements of FIG. 7 similar tocorresponding elements of FIG. 6 are labelled with reference signssimilar to those used in FIG. 6, but with prefix “7” instead of “6”.

The gas delivery element 702 delivers gas 704 in the first direction703. In the present example, the gas delivery element is substantiallyidentical to that showed in FIG. 7, i.e. it comprises a first wall 712having an optical input 706 therein and a second wall 714 having anoptical output 708 therein. A first portion of the gas 720 will escapethrough the optical input 706 and a second portion of the gas 722 willescape through the optical output 708. As explained above, the secondportion of the gas will serve to absorb the generated radiation, whichreduces the intensity of the generated radiation. A third portion of thegas 705 will propagate towards an output of the gas delivery element.

Turning now specifically to FIG. 7(a), which illustrates the knownsituation, the first direction 703 is perpendicular to the seconddirection 710, i.e. the angle 724 a between the first direction and thesecond direction is 90 degrees. Due to this perpendicularity, theportion of gas escaping from the optical input is substantiallyidentical to the portion of gas escaping from the optical output.

FIG. 7(b) illustrates an exemplary situation in accordance with anaspect of the present disclosure. In this exemplary situation, the firstdirection 703 arranged relative to the second direction 710 at an angle724 b that is not perpendicular or parallel. In the exemplary situationof FIG. 7(b), similarly to the situation described in FIG. 7(a), a firstportion of the gas will escape through the optical input and a secondportion of the gas will escape through the optical output. Due to thenon-perpendicularity, the first portion of gas is larger than the secondportion of gas.

FIGS. 7(c)-7(e) illustrate a number of exemplary situationssubstantially similar to the one illustrated in FIG. 7(b), but fordifferent values of the angle 724 c, 724 d, 724 e between the firstdirection and the second direction. As will be appreciated, as the anglebetween the first direction 703 and the second direction 710 increases,the gas flow through the optical input (i.e. the first portion)increases, and the gas flow through the optical output (i.e. the secondportion) decreases. It will be appreciated that the values illustratedbelow are exemplary only so as to illustrate the principles of thepresent disclosure.

Flow through Flow through optical optical output Angle (degrees) input(a.u.) (a.u.) 0 1 1 10 1.09 0.93 30 1.67 0.70 45 2.25 0.52 50 2.83 0.46

The increased gas flow on the input side of the gas delivery elementdoes not substantially affect the output of measurement radiation.However, by reducing the amount of gas that exits the gas deliveryelement through the optical output, the absorption of the soft X-rayradiation due to the presence of particles at the optical output isreduced. In turn, this increases the radiation output of theillumination source.

FIG. 8 shows a second exemplary implementation of a gas delivery systemfor use in an illumination source in accordance with a second aspect ofthe present invention. For ease of comparison with FIG. 6, elements ofFIG. 8 similar to corresponding elements of FIG. 6 are labelled withreference signs similar to those used in FIG. 6, but with prefix “8”instead of “6”.

Similarly to the example described with reference to FIG. 6 above, thegas delivery element 802 comprises an optical input 806 and an opticaloutput 808 that define an optical path. In the present example, theoptical path forms part of a substantially cylindrical radiation guidethat defines the second direction 810. The gas delivery element furthercomprises a substantially toroidal gas delivery component 807. Thetoroidal gas delivery component is connected to the radiation guide byway of at least one gas delivery passage 809, thereby to deliver gas tothe radiation guide in at least a first direction 803. The gas deliverypassage may have any suitable shape. In the present example, the gasdelivery passage has a substantially frustro-conical shape so that thegas is delivered substantially symmetrically around the second direction810. In other terms, the gas delivery passage delivers gas 804 at aplurality of angles that are non-perpendicular and non-parallel to thesecond direction.

It will be appreciated that the implementations discussed above areexemplary only, and that many specific implementations may be envisagedin accordance with the principles of the present disclosure.

As discussed above, it is desirable to maximize the intensity andstability of the measurement radiation. In order to ensure stability, itis necessary to ensure that the supply of gas to the HHG gas cell iskept at a substantially constant level. Any variation or instability ofthe characteristics of the supplied gas, e.g. (but not limited to) gasflow speed or gas flow volume, will cause the characteristics of themeasurement radiation to vary over time. For example, a reduced flow ofgas will reduce the amount of measurement radiation, which reduces theperceived intensity of the measurement radiation. This, in turn, mayinfluence the quality of the measurements performed using themeasurement radiation.

Accordingly, it is advantageous maintain the stability of the gassupplied to the HHG gas cell at a constant and high level. A number ofexemplary implementations will now be discussed that are intended toimprove the stability of gas supply to the gas delivery system.

FIG. 9 illustrates a radiation source 930 comprising an exemplary gasdelivery system. For ease of comparison with FIG. 4, elements of FIG. 9similar to corresponding elements of FIG. 4 are labelled with referencesigns similar to those used in FIG. 4, but with prefix “9” instead of“4”.

The gas delivery system comprises a gas buffer element 960 that ispositioned between the gas source 934 and the HHG gas cell 932. The gasbuffer element has an input that is connected to an output of the gassource, and an output that is connected to an input of the HHG gas cell.In operation, gas is transferred from the gas source (which may be ahigh pressure gas bottle fitted with a simple valve) and into the gasbuffer element. Typically, gas sources comprise outputs that have highlevels of gas flow variation and instability. The gas buffer element isoperable to absorb variations in gas flow from the gas source and todeliver a substantially constant gas flow to the HHG gas cell.

The gas buffer element may have any suitable shape or form. In someexamples, it may be provided with control means 962 for controlling oneor more of the characteristics of the gas. In specific examples, the gasbuffer element comprises a temperature control element. This allows thegas temperature to be controlled, which may reduce the variations of thespecific mass of the gas due to temperature variations of theenvironment at the gas delivery system itself. It will be appreciatedthat, while discussed as being comprised as part of the gas bufferelement in the present example, the temperature control element may bepositioned outside, whilst being used in conjunction with, the gasbuffer element in some examples. In other examples, additional oralternative control means may be provided. In some examples, the gasbuffer element or gas delivery system may comprise a gas compositioncontrol element. This is particular relevant in situations where the gascomprises a mixture of specific gasses or compounds. In yet otherexamples, the gas buffer element or gas delivery system may comprise apurity control element. This is relevant in situations where quality orpurity control of the gas is a concern. In specific examples, the puritycontrol element is coupled to a scrubber or gas replacement element.

FIG. 10 schematically illustrates a number of exemplary gas deliveryelements for improving the flow profile of the gas. For ease ofcomparison with FIG. 6, elements of FIG. 10 similar to correspondingelements of FIG. 6 are labelled with reference signs similar to thoseused in FIG. 6, but with prefix “10” instead of “6”.

FIG. 10(a) shows a known gas delivery element 1002, e.g. a capillarytube such as discussed with reference to FIG. 5. It is known that, in agas delivery element such as the one shown, a moving gas 1004 has aspecific flow profile (as illustrated by the dotted line 1024).Typically, the velocity and pressure of the gas is largest near themiddle of the gas delivery element and is smallest near the walls of thegas delivery element. At the output of the gas delivery element, theflow profile 1026 a is substantially identical to that within the gasdelivery element. Since the flow profile is not transversallyhomogenous, the gas spread on exiting the gas delivery is higher than itwould be if the gas flow profile is substantially transversallyhomogenous (e.g. if the gas flow in the gas delivery element islaminar). As a result, a “cloud” of gas is formed in the area whereinthe gas interacts with incoming pump radiation. In order to maximizeconversion of pump radiation into generated radiation and order tominimize the absorption of the measurement radiation by gas particles,it is desirable to ensure that the gas glow is laminar.

FIG. 10(b) illustrates an exemplary gas delivery element 1002 inaccordance with the present disclosure. It will be appreciated that,while illustrated as a capillary tube substantially identical to the oneshown in FIG. 10(a), the principles of this example may be applied toany number of specific implementations.

The gas delivery element shown in FIG. 10(b) comprises a filteringelement 1028. The filtering element is operable to modify the flowprofile of the gas flowing through the gas delivery element. In someexamples, the filtering element is operable to provide gas with alaminar flow profile 1026 b at the output of the gas delivery element.By reducing the amount of turbulence in the gas, the spread of gas onceit exits the gas delivery element is reduced. This, in turn, maintains ahigher concentration of gas in the area wherein the gas interacts withthe pump radiation (which increases the conversion efficiency and theresulting intensity of the generated radiation). In the present example,the filtering element 1028 comprises a first filter component 1030 and asecond filter component 1032. In this example, both the first filtercomponent and the second filter component comprise a plurality of pores.The first filter component equalizes the pressure and velocity of thegas inside the filtering element. The second filter provides a uniformflow distribution of the gas exiting the gas delivery element. It willbe appreciated that it is possible to control the specific flowdistribution by controlling the pore size and distribution of the gas,for example to optimize the generation of measurement radiation. It willfurther be appreciated that this example is exemplary only, and that anumber of specific implementations may be envisaged that perform therequired function, i.e. providing a laminar flow of fluid in the gasdelivery element. It will further be appreciated that, while illustratedin FIG. 10 as a capillary tube, the principles of this example may bereadily applied to other specific exemplary implementations of thepresent disclosure.

To generate (e.g., soft X-ray) measurement radiation with a reasonableenergy conversion efficiency, several physical parameters can be tuned.One such parameter which has an impact on generation of HHG measurementradiation is the number density of the gas within the gas cell.Preferably, the number density should be high within a pump radiationinteraction region (where pump radiation interacts with/excites the gas)for phase matching and ionization, but low in the region immediatelybeyond this location to prevent measurement radiation absorption.Furthermore, for efficient measurement radiation generation, the highdensity region should extend over a certain minimum distance, e.g., afew millimeters, and then drop off sharply to a low density, e.g.,within 10% of the length of the pump radiation interaction region alongthe direction of the pump radiation beam.

It is therefore proposed to use a gas jet shaping device to shape thegas jet such that the drop-off length from high number density to lownumber density is decreased. The gas jet shaping device may furthershape the gas jet such that length of the pump radiation interactionregion (length of region with high number density) is increased relativeto there being no gas jet shaping device present. In an embodiment, gasjet shaping device may be such that the short drop off length is lessthan 10% of the length of the pump radiation interaction region. “Lowdensity” in this context may comprise the background pressure in thevacuum vessel, which should be sufficiently low so as to not absorbsignificant amount of the measurement radiation, e.g., typically 1-10Pa. In an embodiment, the drop off length may describe the distance overwhich the gas density drops by at least a factor of ten, from thedensity at the pump radiation interaction region to the low densityregion. The actual length of the pump radiation interaction region maybe varied by an order of magnitude or more; the optimal length of thepump radiation interaction region will depend on gas type and pumpradiation beam intensity and focus.

In an embodiment, the gas jet shaping element may comprise an angledwall element located below the gas delivery element. The angled wallelement may be attached to the gas delivery element at a point adjacentits output (e.g., the nozzle output) such that it extends below thisoutput (for example, by a few mm—e.g., less than 10 mm) at an angletowards the emitted gas jet. The angle (relative to the vertical orz-axis) may be, for example, between 20 and 60 degrees, between 20 and50 degrees, or between 30 and 40 degrees. In an embodiment, the gas jetshaping element may comprise an angled horizontal-cylindrical-segment(e.g., a cylindrical segment cut horizontally), open at its bottom end.More specifically, the gas jet shaping element may comprise asemi-cylindrical element. The gas jet shaping element may be locatedsuch that a wall of the gas jet shaping element is located between thegas jet and optical output of the gas cell, and no wall of the gas jetshaping element is located between the gas jet and optical input of thegas cell. The gas shaping element may comprise an aperture to transmitthe generated measurement radiation.

FIG. 11 is a schematic illustration of a gas delivery element 1170 withgas jet shaping element 1180 according to an embodiment, depicted (a) inisometric projection and (b) in cross-section. The gas delivery element1170 comprises nozzle outlet 1184. The gas jet shaping element 1180 inthis embodiment is semi-cylindrical, with its wall located between thegas jet 1104 and optical outlet (not shown). The gas shaping element1180 comprises an aperture 1182 to transmit the generated measurementradiation. The shape and location of the gas shaping element 1180results in a sharp drop-off in the number density of the gas jet 1104,indicated by dotted line 1186. As such, the region along the opticalpath, immediately after this dotted line 1186 in the direction towardsthe optical outlet, has a very low number density (e.g., backgroundpressure in the vacuum vessel). This acts to reduce the absorption ofthe generated measurement radiation by the gas jet 1104. Relatively fewgas molecules go through aperture 1182 due to the high speed and kineticenergy of the gas molecules in the vertical direction.

In an embodiment, the wall of gas jet shaping element 1180 may be thin,for example less than 0.2 mm, or in the region of 0.1 mm. It maycomprise any suitable material, e.g., a weldable material (e.g., so thatit can be welded to a disc 1187 around the nozzle outlet 1184). Thematerial may be wear resistant and have a high melting temperature.Suitable materials may comprise tungsten, molybdenum, aluminum orstainless steel, for example.

The effect of the gas jet shaping element 1180 is to provide for ahigher density gas on the gas delivery side of the gas jet shapingelement 1180, and a steep density drop off at the outlet side of the gasjet shaping element 1180, close to the location of aperture 1182. Morespecifically, the gas jet shaping element 1180 provides for a relativelylonger (relative to there being no gas jet shaping element) pumpradiation interaction region on its gas delivery side and a density dropoff region having a length less than 10% the length of the pumpradiation interaction region on its outlet side.

FIG. 12 illustrates this effect. It shows two gas density profiles,first gas density profile 1291 (solid line) for a gas delivery systemhaving no gas shaping element, and a second gas density profile 1292(dotted line) for a gas delivery system having a gas shaping element asdisclosed herein. In each case the gas density profile comprises a plotof molecular gas density on a logarithmic scale (on the y-axis) againstdistance along the optical path (x-axis) with the origin 0 at the centerof the output nozzle. Also marked on the y-axis is a first gas densitylevel line 1293 indicating a first gas density level at which the pumpradiation will excite the gas. This corresponds to a region ofrelatively high density (e.g., at least 50% of peak) at a particulardistance below the nozzle output; the pump radiation does not go throughthe density peak since the laser interaction length would then be zero.A second gas density level line 1294 indicates the gas density level atwhich the gas density falls by an order of magnitude (i.e., a factor of10) relative to the first gas density level. The second gas densityprofile 1292 shows a longer pump radiation interaction length 1295relative to the pump radiation interaction length 1296 of the first gasdensity profile 1291. Also shown is the drop off length 1297 of thesecond gas density profile 1292 and the drop off length 1298 of thefirst gas density profile 1291. Of particular note is that the gasdensity drop off length 1297 is less than 10% of the pump radiationinteraction length 1295 for the second gas density profile 1292, whilethe gas density drop off length 1298 is greater than 10% of the pumpradiation interaction length 1296 for the first gas density profile1291.

It should be noted that the actual length of the pump radiationinteraction region 1295 is variable depending on gas type, pressure etc.As such, one purely exemplary operation point may have a pump radiationinteraction length 1295 of 1.5 mm and a drop off length 1297 of 0.1 mm.However this can, for example, also be scaled to be 10× smaller, with ahigher gas density. Hence another (purely exemplary) operation point mayhave a pump radiation interaction length 1295 of 0.2 mm and a drop offlength 1297 of <0.02 mm. The gas density profile is scalable in width(x-axis: mm) and height (y-axis: gas density).

It should be noted that the gas jet shaping element as disclosed hereinmay be used in conjunction with the other embodiments disclosed, i.e., agas delivery element with an optical path which is non-perpendicular ornon-parallel to the gas jet delivery direction, or it can beincorporated in a more conventional gas delivery element where theoptical path is perpendicular to the gas jet delivery direction.

As already described, there are advantages in reducing the gas densitydrop off length. Reabsorption by the gas of the generated measurementradiation may be prevented substantially where the gas density drop offof the gas medium has a sharp edge in the propagation direction x of thepump radiation, such that the gas density drops off very sharplyimmediately after the pump radiation interaction length. Morespecifically, such a sharp edge may be at a position x=x0 along thepropagation axis x of the pump radiation beam at which the gas densitysharply drops from the specific phase-matched density at x<x0 to adensity at x>x0 which is sufficiently low for the gas to be essentiallytransparent for soft X-ray radiation. The soft X-ray radiation intensitytypically reaches a high peak value near the focus of the pump radiationbeam. Should the gas density profile slowly drop following this maximum(as is typical), the soft X-ray intensity will decrease to a low valuedue to its absorption by the gas in the region x>x0. In contrast, wherethe gas density has a sharp edge near x=x0, absorption in the regionx>x0 is substantially avoided and the soft X-ray intensity remains highin the region x>x0. The degree of ‘sharpness’ of the gas density profileresults in this desired behavior when the density drops over a lengththat is comparable to or smaller than the absorption length of the gas,which is a characteristic length scale dependent on the gas species.Typically, absorption lengths of gases that are suitable for soft X-raygeneration at wavelengths below 20 nm are on the order of some tenths ofmillimeters or smaller. The gas density is typically in the range1023-1026 m−3.

It is proposed to use a second high intensity (e.g., laser) radiationpulse to generate a highly or fully ionized plasma in the region x>x0 atthe moment in time that the soft X-ray pulse passes this region. Aplasma is comprised of ions which have a much smaller cross-section forabsorption than the original atoms (or molecules) of the gas phase, andtherefore will absorb significantly less soft X-ray measurementradiation. This avoids the need for a steep particle density gradientand provides additional control over the exact region in which themeasurement radiation is generated and re-absorbed.

A laser intensity of typically ≳1018 W/m2 may be used to generate ahighly ionized plasma. To generate this over a length of approximately 3mm (the length over which the gas density drops significantly in a gasjet configuration) a laser pulse with a wavelength of e.g. 1 μm could beused, focused to a spot of approximately 30 μm diameter. To reach theintensity of 1018 W/m2, the energy of a 50 fs laser pulse would be ofthe order 0.5 mJ, which is comparable to the pump laser pulse forgenerating the measurement radiation.

FIG. 13 shows a possible configuration comprising a counter-propagatinglaser pulse, which illustrates (a)-(d) the pump radiation pulse 1300 andcounter-propagating ionization radiation pulse 1310 at different timesduring the measurement radiation generation process. FIG. 13(a) showsthe pump radiation pulse 1300 and ionization radiation pulse 1310 at afirst time, each propagating towards the gas target 1320 at x=x0. FIG.13(b) shows the pump radiation pulse 1300 and ionization radiation pulse1310 at a later time, each getting nearer to the focus with increasedintensity. FIG. 13(c) shows the plasma 1330 being generated as a resultof the ionization radiation pulse ionizing the gas target 1320 in theregion immediately after x=x0 relative to the pump radiation propagationdirection (e.g., x>x0). The delay between the pulses is tuned such thatthey achieve overlap at (close to) x=x0, the position where thegenerated measurement radiation intensity reaches its maximum. Thegenerated soft X-ray measurement radiation 1340 therefore propagatesthrough plasma 1330, rather than neutral gas. FIG. 13(d) shows thegenerated soft X-ray measurement radiation 1340 co-propagating with thepump radiation, subsequent to having propagated through plasma 1330.

The ionization radiation pulse can be synchronized with the pumpradiation pulse. This will be automatically the case if the ionizationradiation pulse is generated from the same oscillator (e.g., pump laser)as the pump radiation pulse. The delay of the ionization radiation pulsemay be tuned such that the plasma is formed at the moment the pumpradiation pulse arrives at x=x0 to generate the measurement radiation.The skilled person will recognize that this is reasonablystraightforward to achieve with a micrometer and sub-picosecondprecision. Accurate timing also means that the position at which themeasurement radiation pulses encounter the gas/plasma boundary can beactively chosen. The counter-propagating ionization pulse can be of thesame wavelength as the pump laser pulse, or another wavelength. Thefocus size (and therefore the divergence) of the ionization radiationpulse may be similar, but not necessarily the same, as that of the pumpradiation pulse. The focus size and divergence of the ionizationradiation can be matched to the size of the generated measurementradiation beam, so that the measurement radiation beam propagatesthrough the highly ionized plasma, instead of the non-ionized gas.

FIG. 14 shows a possible arrangement for generating thecounter-propagating pump radiation pulse 1400 and ionization radiationpulse 1410 from a single radiation source (oscillator) 1405. The outputradiation 1407 is split by beamsplitter 1415 into pump radiation beampath 1425 and ionization radiation beam path 1427. The pump radiation isdirected to the gas target 1420 by optical elements 1435. The ionizationradiation is directed to the gas target 1420 by optical elements 1450and via delay stage 1430. While the delay stage 1430 is shown here inthe ionization radiation beam path 1427, it can be located in eitherbeam path, or even both beam paths.

To ensure that the returning ionization radiation pulse is preventedfrom re-entering the laser source system 1405, a number of ways ofseparating the laser pulses is possible. Shown here is an arrangementwhich uses polarization to separate the pulses. A half-wave plate 1455is located in one of the beam paths (it does not matter which), followedby polarizing beamsplitters 1460, 1475 and beam dumps 1470, 1485 in eachbeam path. Due to this arrangement, the two pulses 1400, 1410 will haveorthogonal polarization, and therefore the incoming radiation pulses canbe separated from the returning radiation pulses by means of thepolarizing beamsplitters 1460, 1475, with the incoming pulses directedto the beam dumps 1470, 1485. Alternatively, the ionization radiationpulses may be introduced into the system at a slight angle, to separatethe beam path of the returning pulses from the beam path of the incomingpulses. As another alternative, different wavelengths may be used forthe pulses, which allows separation by dichroic mirrors, or filters.

FIG. 15(a)-(c) shows, at three different times, an alternative possibleconfiguration for generating the plasma. This uses a cylindrical lens1550, cylindrical mirror or other suitable optical element to focus theionization radiation pulse 1560 into a line focus. This line is orientedon the path of the measurement radiation pulses 1540, (e.g., in theregion immediately after x=x0 relative to the pump radiation pulse 1500propagation direction) such that the generated measurement radiationpulses propagate through plasma 1530 instead of neutral gas atoms. Anadvantage of this configuration is that a longer plasma can begenerated, relative to the FIG. 13 arrangement. Also, there is a greaterde-coupling of timing from position. The plasma can be generated at atime (in the order of picoseconds, up to nanoseconds) before themeasurement radiation pulses arrive. This avoids the need for accuratetiming.

It is not necessary that the ionization radiation pulse arrives fromeither the opposite direction or an orthogonal direction, relative tothe propagation direction of the pump radiation pulse, as isspecifically shown in FIGS. 13 and 15. In other embodiments thepropagation directions of the ionization radiation pulse and pumpradiation pulse may be separated by a different angle. What is relevantis that the generated plasma is adjacent to the area where the generatedmeasurement radiation pulses are generated and such that thesemeasurement radiation pulses have (at most) only a short distance topropagate through the high harmonic generation gas before encounteringthe plasma.

A further advantage of the plasma generation methods disclosed above isin dose control. By accurately timing the ionization radiation pulsewith respect to the pump radiation pulse, and/or by controlling theintensity of the ionization radiation pulse, it is possible to increaseor decrease the absorption by the neutral gas. As such, the proposedillumination source may comprise a dose control which controls thetiming the ionization radiation pulse with respect to the pump radiationpulse, and/or by controlling the intensity of the ionization radiationpulse. This dose control can also be used to control the dose leveland/or effectively temporarily ‘switch off’ the measurement radiationpulses by ensuring that no plasma is produced. If no plasma is producedand the gas configuration is chosen appropriately, all or nearly allmeasurement radiation will be reabsorbed by the gas. The dose controlcan be implemented in any suitable manner, e.g., by suitable softwarerunning on a processor which controls the ionization radiation pulse andgeneration thereof. Such a processor may be a processor which controlsoperation of the illumination source more generally.

The plasma generated by the ionization radiation source will refract thepump radiation pulse whereas the measurement radiation pulse is onlynegligibly affected. This is because the frequency of the measurementradiation pulse is much higher than the plasma frequency in the relevanttypical density range. This means that it will be easier to separate thepump radiation pulses from the soft X-ray measurement radiation, e.g.,by means of a small pinhole in the beam path aligned with themeasurement radiation beam but which blocks the refracted, and thereforedeflected, pump radiation pulse.

It should be noted that the gas ionization method as disclosed hereinmay be used in conjunction with the other embodiments disclosed, i.e., agas delivery element with an optical path which is non-perpendicular ornon-parallel to the gas jet delivery direction, and/or gas jet shapingelement delivery element as shown in FIG. 11 and/or the provision of twoor more gas nozzles described below.

FIG. 16 illustrates an alternative arrangement for mitigating for thereabsorption of the measurement radiation by the gas from which it isgenerated. In this arrangement, it is proposed to use a at least two gasjets with two different gases (e.g., different gas species). FIG. 16(a)illustrates such an arrangement, with first gas nozzle 1600 a and secondgas nozzle 1600 b. The gases can be chosen so that the first gas jet1610 a emitted by the first gas nozzle 1600 a comprises the gas mediumwhich efficiently produces the desired (e.g., soft X-ray) measurementradiation. As such, this first gas may be the gas used in the otherembodiments described above. A second gas jet 1610 b emitted by thesecond gas jet nozzle 1600 b may comprise a second gas which has a muchlower absorption at the desired (e.g., soft X-ray) wavelength. Forexample, if the measurement radiation generated comprises soft X-rayradiation in the 10-20 nm range, then neon may be a suitable first gasand argon may be a suitable second gas. Argon has a much lowerabsorption at that wavelength range than neon.

The second gas jet 1610 b flow acts to shape the first gas jet 1610 a toobtain a steep density drop off gradient immediately after the x=x0position. This is illustrated in FIG. 16(b), which is a plot of gasdensity against distance in x (pump radiation propagation direction).The first plot 1620 (thinner line) is the density profile for the firstgas and the second plot 1630 (thicker line) is the density profile forthe second gas. Also shown (dotted line) is a plot of the overall gasdensity 1640 within the gas target region (HHG gas cell). It can be seenthat the density profile 1620 for the first gas shows a very steepdrop-off at the x=x0 position. Beyond this position is essentially onlythe second gas which will not absorb the measurement radiation.

In the embodiment shown in FIG. 16(a) the gas jets are adjacent eachother. Optionally, the second gas jet 1610 b may be tilted towards thefirst gas jet 1610 a. In another possible embodiment, the gas jets maybe arranged concentrically; e.g., with the first gas nozzle locatedinside the second gas nozzle.

More gas jets, having different gas profiles may be added to shape thegas profile of the first gas. The gas jets can be operated at similar orat different gas pressures. They can also be of different shape andsize.

It should be noted that the two or more gas nozzles embodiment asdisclosed herein may be used in conjunction with the other embodimentsdisclosed, i.e., a gas delivery element with an optical path which isnon-perpendicular or non-parallel to the gas jet delivery direction,and/or gas jet shaping element delivery element as shown in FIG. 11and/or the ionization pluses to generate a plasma method describedabove.

Further embodiments are defined in the subsequent numbered clauses:

1. A gas delivery system for use in an illumination source, comprising agas delivery element arranged to direct gas in at least a firstdirection, wherein the gas delivery element comprises:

-   -   an optical input; and    -   an optical output,    -   wherein the input and the output define an optical path, the        optical path being oriented in a second direction, and

wherein the second direction is non-perpendicular and non-parallel tothe first direction.

2. A gas delivery system according to clause 1, wherein the firstdirection is at an obtuse angle relative to the second direction.

3. A gas delivery system according to clause 1 or 2, wherein the opticalinput and optical output are arranged to allow pump radiation to passthrough the gas to generate high harmonic radiation.

4. A gas delivery system according to clause 3, wherein the opticalinput and optical input are arranged concentrically with the opticalpath.

5. A gas delivery system according to clause 3 or 4, wherein the opticalinput comprises an opening in a first wall of the gas delivery system,and wherein the optical output comprises an opening in an opposing wallof the gas delivery system.

6. A gas delivery system according to any preceding clause, wherein atleast one of the optical input or the optical output has a cross sectioncorresponding substantially to a beam cross section of the pumpradiation.

7. A gas delivery system according to any preceding clause, furthercomprising at least one pumping element connected to the gas deliveryelement.

8. A gas delivery system according to any preceding clause, wherein thegas delivery element has a cross-section in the first direction that isone of: rectangular; circular; or ellipsoidal.

9. A gas delivery system according to any of clauses 1 to 7, wherein thegas delivery element comprises a toroidal gas delivery component, thetoroidal gas delivery component being arranged to deliver gas to theoptical path in at least the first direction.

10. A gas delivery system according to clause 9, wherein the toroidalgas delivery component is arranged to deliver gas to the optical in aplurality of first directions.

11. A gas delivery system according to any preceding clause, furthercomprising a gas buffer element.

12. A gas delivery system according to clause 11, wherein the gas bufferelement comprises a temperature controlling element.

13. A gas delivery system according to any preceding clause, furthercomprising a filtering element operable to modify a flow profile of thegas.

14. A gas delivery system according to clause 13, wherein the filteringelement is operable to provide a laminar flow profile of the gas.

15. A gas delivery system according to any preceding clause, furthercomprising a gas jet shaping device operable to modify a flow profile ofthe gas such that number density of the gas falls sharply in thedirection of the optical output, after a pump radiation interactionregion where pump radiation interacts with said gas.

16. A gas delivery system according to clause 15, wherein the numberdensity of the gas falls by at least a factor of 10 relative to that ofthe pump radiation interaction region within a drop off regionimmediately after the pump radiation interaction region in the directiontoward said output, a length of said drop off region being 10% orsmaller than a length of the pump radiation interaction region.

17. A gas delivery system according to clause 15 or 16, wherein saidmodification of the flow profile is further operable to extend a lengthof the pump radiation interaction region relative to there being no gasjet shaping device present.

18. A gas delivery system according to clause 17, wherein said length ofthe pump radiation interaction region is extended by more than 50%relative to there being no gas jet shaping device present.

19. A gas delivery system according to any of clauses 15 to 18, whereinthe gas jet shaping device comprises an angled wall element locatedbelow the gas delivery element and obtusely angled relative to saidfirst direction.

20. A gas delivery system according to clause 19, wherein the angledwall element is attached at a point adjacent a gas output of the gasdelivery element such that it extends below this gas output and at anangle towards the gas emitted.

21. A gas delivery system according to clause 20, wherein the gas jetshaping element comprises an angled horizontal-cylindrical-segment openat its bottom end.

22. A gas delivery system according to clause 21, wherein the gas jetshaping element comprises a semi-cylindrical element.

23. A gas delivery system according to any of clauses 19 to 22, whereinthe gas jet shaping element is located such that a wall of the gas jetshaping element is located between the pump radiation interaction regionand the optical output, and no wall of the gas jet shaping element islocated between the gas pump radiation interaction region and theoptical input.

24. A gas delivery system according to clause 23, wherein the gasshaping element comprises an aperture in said wall to pass the generatedmeasurement radiation to said optical output.

25. A gas delivery system for use in an illumination source, comprising:

a gas delivery element arranged to direct gas in at least a firstdirection, wherein the gas delivery element comprises:

an optical input and an optical output together defining an opticalpath, the optical path being oriented in a second direction; and

a gas jet shaping device operable to modify a flow profile of the gassuch that number density of the gas falls sharply in the direction ofthe optical output after a pump radiation interaction region where pumpradiation interacts with said gas.

26. A gas delivery system according to clause 25, wherein the numberdensity of the gas falls by at least a factor of 10 relative to that ofthe pump radiation interaction region within a drop off regionimmediately after the pump radiation interaction region in the directiontoward said output, a length of said drop off region being 10% orsmaller than a length of the pump radiation interaction region.

27. A gas delivery system according to clause 25 or 26, wherein saidmodification of the flow profile is further operable to extend a lengthof the pump radiation interaction region relative to there being no gasjet shaping device present.

28. A gas delivery system according to clause 27, wherein said length ofthe pump radiation interaction region is extended by more than 50%relative to there being no gas jet shaping device present.

29. A gas delivery system according to any of clauses 25 to 28, whereinthe gas jet shaping device comprises an angled wall element locatedbelow the gas delivery element and obtusely angled relative to saidfirst direction.

30. A gas delivery system according to clause 29, wherein the angledwall element is attached at a point adjacent a gas output of the gasdelivery element such that it extends below this gas output and at anangle towards the gas emitted.

31. A gas delivery system according to clause 30, wherein the gas jetshaping element comprises an angled horizontal-cylindrical-segment openat its bottom end.

32. A gas delivery system according to clause 31, wherein the gas jetshaping element comprises a semi-cylindrical element.

33. A gas delivery system according to any of clauses 29 to 32, whereinthe gas jet shaping element is located such that a wall of the gas jetshaping element is located between the pump radiation interaction regionand the optical output, and no wall of the gas jet shaping element islocated between the gas pump radiation interaction region and theoptical input.

34. A gas delivery system according to clause 33, wherein the gasshaping element comprises an aperture in said wall to pass the generatedmeasurement radiation to said optical output.

35. An illumination source for generating high harmonic radiation,comprising:

a pump radiation source operable to emit pump radiation; and

a gas delivery system as claimed in any of clauses 1 to 34 or 39 to 43,operable to receive the emitted pump radiation and to generate said highharmonic radiation.

36. An inspection apparatus for measuring a target structure on asubstrate, comprising:

an illumination source as claimed in clause 35 for generating highharmonic radiation; and

a sensing element for receiving high harmonic radiation scattered by thetarget structure.

37. A lithographic apparatus comprising an illumination optical systemarranged to illuminate a pattern, and a projection optical systemarranged to project an image of the pattern onto a substrate,

wherein the lithographic apparatus comprises an illumination source asclaimed in clause 35 or any of clauses 44 to 55.

38. A lithographic system comprising:

a lithographic apparatus comprising an illumination optical systemarranged to illuminate a pattern, and a projection optical systemarranged to project an image of the pattern onto a substrate; and

an inspection apparatus as claimed in clause 36,

wherein the lithographic apparatus is arranged to use one or moreparameters calculated by the inspection apparatus in applying thepattern to further substrates.

39. A gas delivery system for use in an illumination source, comprisingat least a first gas delivery element operable to emit a first gas and asecond gas delivery element operable to emit a second gas in such a waythat a number density profile of the first gas is altered by the secondgas.

40. A gas delivery system as claimed in clause 39, wherein the first gasis a high harmonic generation gas medium for the generation of highharmonic radiation and the second gas has a lower absorption of highharmonic radiation than the first gas.

41. A gas delivery system as claimed in clause 39 or 40, wherein thesecond gas is operable to modify a flow profile of the first gas suchthat number density of the gas falls sharply in the propagationdirection of a pump radiation pulse after a pump radiation interactionregion where the pump radiation pulse interacts with said first gas.

42. A gas delivery system as claimed in clause 39, 40 or 41, wherein thefirst gas delivery element is adjacent the second gas delivery element.

43. A gas delivery system as claimed in clause 39, 40 or 41, wherein thefirst gas delivery element and the second gas delivery element arearranged concentrically.

44. An illumination source for generating high harmonic radiation, theillumination system comprises the gas delivery system according to anyone of the clauses 39 to 43 and comprising a pump radiation sourceoperable to emit pump radiation at the first gas.

45. An illumination source for generating high harmonic radiation,comprising:

a pump radiation source operable to emit pump radiation at a highharmonic generation gas medium thereby exciting said high harmonicgeneration gas medium within a pump radiation interaction region so asto generate said high harmonic radiation; and

an ionization radiation source operable to emit ionization radiation atthe high harmonic generation gas medium to ionize said gas at anionization region between the pump radiation interaction region and anoptical output of the illumination source.

46. The illumination source of clause 45, wherein the ionization regionis immediately adjacent said pump radiation interaction region.

47. The illumination source of clause 45 or 46, wherein the illuminationsource is arranged such that the ionization radiation ionizes said gasat the ionization region substantially simultaneously with the pumpradiation exciting said high harmonic generation gas medium.

48. The illumination source of clause 45, 46 or 47, wherein thepropagation direction of the ionization radiation is opposite to thepropagation direction of the pump radiation.

49. The illumination source of clause 48, wherein the pump radiationsource and the ionization radiation source are located on opposite sidesof high harmonic generation gas medium.

50. The illumination source of clause 49, operable such that at leastone of the wavelength, polarization or propagation angle of the pumpradiation is different to that of the ionization radiation, therebyenabling the separation of returning pump radiation and/or ionizingradiation.

51. The illumination source of clause 45, 46 or 47, wherein thepropagation direction of the ionization radiation is orthogonal to thepropagation direction of the pump radiation.

52. The illumination source of clause 51, comprising an optical elementoperable to focus the ionization radiation on a line focus at theionization region.

53. The illumination source of any of clauses 45 to 52, comprising acommon oscillator operable to provide both the pump radiation andionization radiation.

54. The illumination source of any of clauses 45 to 53, wherein saidoptical output comprises an aperture arranged to allow passage the pumpradiation and to block the high harmonic radiation, the pump radiationhaving properties such that it is subject to greater deflection byrefraction within the ionization radiation than the high harmonicradiation.

55. The illumination source of any of clauses 45 to 54, comprising adose control which controls timing of the ionization radiation withrespect to the pump radiation, and/or the intensity of the ionizationradiation, thereby controlling the absorption characteristics within theionization region.

If in this document the term “metrology apparatus” is used, one may alsoread the term “inspection apparatus” at that position, and vice versa.In the context of this document said apparatuses can be used todetermine characteristics of interest of a structure on a substrate. Thecharacteristics of interest may be measurement values and may also bedeviations from an expected pattern, such as the absence of structures,the presence of unexpected structures and changes in the expectedpattern.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

What is claimed is:
 1. An illumination source for generating highharmonic radiation, comprising: a pump radiation source operable to emitpump radiation at a high harmonic generation gas medium thereby excitingthe high harmonic generation gas medium within a pump radiationinteraction region so as to generate the high harmonic radiation; and anionization radiation source operable to emit ionization radiation at thehigh harmonic generation gas medium to ionize a gas at an ionizationregion between the pump radiation interaction region and an opticaloutput of the illumination source.
 2. The illumination source of claim1, wherein the ionization region is adjacent to the pump radiationinteraction region.
 3. The illumination source of claim 1, wherein theillumination source is arranged such that the ionization radiationionizes the gas at the ionization region substantially simultaneouslywith the pump radiation exciting the high harmonic generation gasmedium.
 4. The illumination source of claim 1, wherein a propagationdirection of the ionization radiation is opposite to a propagationdirection of the pump radiation.
 5. The illumination source of claim 1,wherein a propagation direction of the ionization radiation isorthogonal to a propagation direction of the pump radiation.
 6. Theillumination source of claim 1, wherein the pump radiation source andthe ionization radiation source are located on opposite sides of thehigh harmonic generation gas medium.
 7. The illumination source of claim1, wherein at least one of wavelength, polarization and propagationangle of the pump radiation is different to that of the ionizationradiation, thereby enabling a separation of returning pump radiationand/or ionizing radiation.
 8. The illumination source of claim 1,further comprising an optical element operable to focus the ionizationradiation on a line focus at the ionization region.
 9. The illuminationsource of claim 1, further comprising a common oscillator operable toprovide both the pump radiation and ionization radiation.
 10. Theillumination source of claim 1, wherein the optical output comprises anaperture arranged to allow the pump radiation to pass and to block thehigh harmonic radiation.
 11. The illumination source of claim 1, furthercomprising a dose control operable to control an absorption of theionization radiation within the ionization region by controlling atiming of the ionization radiation with respect to the pump radiation,and/or an intensity of the ionization radiation.
 12. An illuminationsource for generating high harmonic radiation, comprising: a pumpradiation source operable to emit pump radiation; and a gas deliverysystem operable to generate a gas flow including a first gas and asecond gas has a lower absorption of high harmonic radiation than thefirst gas; wherein the gas delivery system further comprises an opticalpath to allow the pump radiation to pass through the gas flow to excitethe first gas to generate the high harmonic radiation.
 13. Theillumination source of claim 12, wherein the gas delivery system furthercomprises a filtering element operable to provide a laminar flow profileof the gas flow.
 14. The illumination source of claim 12, wherein thegas delivery system further comprises a gas jet shaping device operableto enlarge a pump radiation interaction region where pump radiationinteracts with the first gas.
 15. The illumination source of claim 14,wherein the gas jet shaping device comprises a curved wall attachedadjacent to a gas output of the gas flow, and an optical aperture on thecurved wall.
 16. The illumination source of claim 12, wherein the gasdelivery system further comprises a toroidal gas delivery componentconfigured to deliver the gas flow to the optical path.
 17. Theillumination source of claim 12, wherein the gas delivery system furthercomprises a gas buffer element operable to maintain the gas flowsubstantially constant.
 18. The illumination source of claim 17, whereinthe gas buffer element comprises a temperature controlling elementoperable to control a temperature of the gas flow.
 19. An illuminationsource for generating high harmonic radiation, comprising: a pumpradiation source operable to emit pump radiation; and a gas deliverysystem operable to generate a gas flow including a first gas and asecond gas configured to modify a density profile of the first gas;wherein the gas delivery system further comprises an optical path toallow the pump radiation to pass through the gas flow to excite thefirst gas to generate the high harmonic radiation.
 20. The illuminationsource of claim 19, wherein the gas delivery system comprises: a firstgas delivery element operable to emit the first gas; and a second gasdelivery element operable to emit the second gas; wherein the first gasdelivery element and the second gas delivery element are arrangedconcentrically.